The present invention relates to a method for testing a bar winding of a rotor of a rotating or linearly moved electrical machine, wherein thermal radiation of the rotor is detected by means of a thermal radiation sensor arranged in a stator or the rotating electrical machine. The invention also relates to a device for testing a bar winding of a rotor of a rotating electrical machine, having a thermal radiation sensor, arranged in a stator of the rotating electrical machine, by means of which thermal radiation of the rotor can be detected.
An electrical machine is a device which converts electrical energy into mechanical energy, in particular kinetic energy (motor operation) and/or mechanical energy into electrical energy (generator operation). A rotating electrical machine is an electrical machine in which a stator provides a, usually, circular opening in which a rotor is arranged so as to be rotatable. The stator is non-rotatably arranged with respect to the rotor. The stator and rotor are linked by means of a magnetic flux, whereby a force effect is generated in motor operation which drives the rotor so as to rotate with respect to the stator, and mechanical energy supplied to the rotor in generator operation is converted into electrical energy. For this purpose the stator and rotor each have a winding through which a current flows. The winding can also be formed or supplemented by a permanent magnet in the stator or rotor.
Rotating electrical machines of the generic type are known from the prior art, so separate documentary evidence is not necessary. Rotating electrical machines of the generic type are by way of example polyphase machines which are connected to a multi-phase, in particular three-phase electrical grid, such as asynchronous machines, synchronous machines with damper cage or the like. The bar winding can in particular be a cage winding, as is formed by way of example by a cage made from bars and shirt-circuit rings for example in the case of a cage rotor of an asynchronous machine. The bar winding preferably comprises conductor bars which extend substantially in an axial direction of the rotor and are electroconductively connected to the rotor-side ends, by way of example by short-circuit rings or end connectors.
The rotors of generic electrical machines are heavily stressed thermomechanically in non-stationary operating conditions. Operating conditions of this kind can be by way of example due to machine start-ups under overcurrent conditions through to attaining the nominal point, blocking of the electrical machine during operation and the like. High currents and power losses in the rotor bars and short-circuit ring can occur in the process. If the electrical machine is then switched off, the dissipated heat is retained in the rotor since the cooling effect of the switched-off machine usually does not exist. This situation occurs in particular with self-cooling in which the rotor simultaneously also drives a cooling system. The thermal energy of the rotor in particular can only be dissipated very inadequately due to the machine construction if no further cooling effect is provided. The rotor can heat up considerably after the electrical machine has been switched off in particular in the case of repeated loads, intermittent operation or the like, with the exact level of heating usually not being known. This can lead to a thermal overloading in particular in the case of renewed start-up of the electrical machine.
The heat is produced in the electrical machine during operation thereof substantially by the following sources of losses: current heat losses, magnetic reversal losses and friction losses. The heat generated by the sources of losses leads to thermal stress on the electrical machine. This stress manifests itself in particular in cyclical expansion and contraction of the machine parts, in particular of the rotor. The current heat losses affect the rotor bars in particular, which expand thermally precisely in the longitudinal direction. Connection points of the rotor bars, by way of example with a short-circuit ring, are mechanically loaded thereby. As a rule, connection points of this kind are formed by soldering, welding or casting. The alternating stress leads to ageing phenomena at the connection points which manifest themselves by way of example in increased electrical resistance in the region of the connection point. The ageing can lead to breaking of the connection point, whereby the electrical machine is ultimately unusable.
Different electrical bar resistances and transition resistances at the connection points also lead to imbalances in the current distribution in the rotor, whereby local temperature differences can occur at the surface of the rotor and moment swings may even be generated.